Outbound calls placed through GV are counted towards your billable minutes. If you are using GV to call other VRZ mobile phones it will not be a mobile to mobile free call, it will come out of their billable minutes as well. Incoming calls to your GV number will be billable and not count as mobile to mobile. But...
If you have Friends and Family you can add your GV number to that and get free incoming calls. You can then get an app like 'Google Voice Callback' and make free outgoing calls (it calls you back at your GV number so it looks like an incoming call). This won't help for mobile to mobile though, you would have to use your VRZ number to make those calls in order for them to be mobile to mobile and free for your VRZ friends.
I also use Voice Choice, it allows me to choose what numbers to always use GV and what numbers to never use GV. Some of those numbers are then set up to go through Google Voice Callback and some aren't. So, some of my outgoing calls are free, some just use GV and are billable and the rest use my cell number and are either billable or mobile to mobile (or FF). My friends who have VRZ use my VRZ number, my friends who have landlines or other mobile carriers use my GV number. ALL of them text me on my GV number. It sounds complicated but it isn't once you get it all set up.
Just remember, the number that shows up on caller ID is what determines if it is mobile to mobile or billable...not what device you use to make or receive the call. That being said, you can have your GV number set to display or not...so make sure it's set correctly (mine is set to show GV so I don't always know who is calling, but it's free!).